12.04 - Failed to fetch repositories; a particular case

This is the question with many answers but I am not sure how to apply them. I please for the particular solution: what to add or remove by changing to comment in the following files and what to check or uncheck in the following dialog windows.
Now, the Ubuntu 12.04 Update Manager tells: "Software updates may be available for your computer.
The package information was last updated 61 days ago.
Press the 'Check' button below to check for new software updates." Clicking at the 'Check' button results in a warning "Failed to download repository information.
Check your internet connection." with Details

W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-proposed/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/restricted/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-security/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-updates/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/universe/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/main/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/multiverse/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-proposed/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/restricted/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-security/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-updates/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/universe/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/main/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/multiverse/binary-i386/Packages  404  Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.

The Internet connection is OK. My /etc/apt/sources.list contains

# deb cdrom:[Ubuntu 12.04.2 LTS Precise Pangolin - Release i386 (20130213)]/ precise main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://cz.archive.ubuntu.com/ubuntu/ precise main restricted
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://cz.archive.ubuntu.com/ubuntu/ precise-updates main restricted
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://cz.archive.ubuntu.com/ubuntu/ precise universe
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise universe
deb http://cz.archive.ubuntu.com/ubuntu/ precise-updates universe
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://cz.archive.ubuntu.com/ubuntu/ precise multiverse
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise multiverse
deb http://cz.archive.ubuntu.com/ubuntu/ precise-updates multiverse
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://cz.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu precise-security main restricted
deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
deb http://security.ubuntu.com/ubuntu precise-security universe
deb-src http://security.ubuntu.com/ubuntu precise-security universe
deb http://security.ubuntu.com/ubuntu precise-security multiverse
deb-src http://security.ubuntu.com/ubuntu precise-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main
deb http://ddebs.ubuntu.com $(lsb_release -cs)-proposed restricted -cs)-security -cs)-updates -cs) universe main multiverse
deb-src http://ddebs.ubuntu.com $(lsb_release -cs)-proposed restricted -cs)-security -cs)-updates -cs) universe main multiverse

My /etc/apt/sources.list.d/precise-partner.list contains

deb http://archive.canonical.com/ubuntu precise partner #Added by software-center

.
My /etc/apt/sources.list.d/webupd8team-y-ppa-manager-precise.list contains

deb http://ppa.launchpad.net/webupd8team/y-ppa-manager/ubuntu precise main
deb-src http://ppa.launchpad.net/webupd8team/y-ppa-manager/ubuntu precise main

and further files in the directory do not seem to contain anything relevant for the failure.
The Ubuntu Software Center...Edit....'Software Sources' shows a window pictured at following screenshots: "http://tpeceny.nazory.cz@tpeceny.nazory.cz/soubory_k_Ubuntu/Software%20Sources1.png"
"http://tpeceny.nazory.cz@tpeceny.nazory.cz/soubory_k_Ubuntu/Software%20Sources2.png"
"http://tpeceny.nazory.cz@tpeceny.nazory.cz/soubory_k_Ubuntu/Software%20Sources3.png"
"http://tpeceny.nazory.cz@tpeceny.nazory.cz/soubory_k_Ubuntu/Software%20Sources4.png".
I have a further PC, almost identical, with the same Ubuntu software. The problem with such update failures at the second PC has been solved by mere clicking at some control at its Update Manager. Probably I have missed the opportunity with the PC described above and I do not know how to repair it. Thanks!

I am editing it after the 'mark kirby's answer and my comment to be able to show clearly the steps.
The /etc/apt/sources.list file from the second PC contains two more lines

deb ddebs.ubuntu.com $(lsb_release -cs)-proposed restricted -cs)-security -cs)-updates -cs) universe main multiverse
deb-src ddebs.ubuntu.com $(lsb_release -cs)-proposed restricted -cs)-security -cs)-updates -cs) universe main multiverse

. After using it the Update Manager says 'The software on this computer is up to date.'.
The /etc/apt/sources.list file resulting from the steps in the answer is different; it contains

# deb cdrom:[Ubuntu 12.04.2 LTS Precise Pangolin - Release i386 (20130213)]/ precise main restricted
deb-src http://archive.ubuntu.com/ubuntu precise main restricted #Added by software-properties
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://cz.archive.ubuntu.com/ubuntu/ precise main restricted universe
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise restricted main multiverse universe #Added by software-properties
## Major bug fix updates produced after the final release of the
## distribution.
deb http://cz.archive.ubuntu.com/ubuntu/ precise-updates main restricted universe
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise-updates restricted main multiverse universe #Added by software-properties
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://cz.archive.ubuntu.com/ubuntu/ precise multiverse
deb http://cz.archive.ubuntu.com/ubuntu/ precise-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://cz.archive.ubuntu.com/ubuntu/ precise-backports main restricted multiverse universe
deb-src http://cz.archive.ubuntu.com/ubuntu/ precise-backports main restricted multiverse universe #Added by software-properties
deb http://security.ubuntu.com/ubuntu precise-security main restricted universe
deb-src http://security.ubuntu.com/ubuntu precise-security restricted main multiverse universe #Added by software-properties
deb http://security.ubuntu.com/ubuntu precise-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu precise partner
# deb-src http://archive.canonical.com/ubuntu precise partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main
deb http://ddebs.ubuntu.com $(lsb_release -cs)-proposed restricted -cs)-security -cs)-updates -cs) universe main multiverse
deb-src http://ddebs.ubuntu.com $(lsb_release -cs)-proposed restricted -cs)-security -cs)-updates -cs) universe main multiverse

. In the Update Manager, clicking at the 'Check' button results in a warning "Failed to download repository information.
Check your internet connection." with Details

W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-proposed/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/restricted/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-security/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-updates/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/universe/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/main/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/multiverse/source/Sources  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-proposed/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/restricted/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-security/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)-updates/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/-cs)/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/universe/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/main/binary-i386/Packages  404  Not Found
, W:Failed to fetch http://ddebs.ubuntu.com/dists/$(lsb_release/multiverse/binary-i386/Packages  404  Not Found
, E:Some index files failed to download. They have been ignored, or old ones used instead.

Have I done a mistake or should I use the /etc/apt/sources.list file from the second PC? Thanks!

You sources are messed up but we can make a new list

Open a terminal and type

sudo rm /etc/apt/sources.list

Then open this

 sudo software-properties-gtk

You will see 5 check boxes tick them all and underneth change the server to the nearest on to you, apliy the options and close the window.

Then go to your terminal and

sudo apt-get update && upgrade

Will update you system